NORD Drivesystems BU0710 - Manual (Page 14)

NORDAC SK 700E manual Posicon special extension
14 BU 0710 GB
3.2.1.1 Tracking the point of reference
Reference point tracking is started when one of the 6 digital inputs provided on the Posicon card or one of the
digital inputs of the customer interface is energized. Accordingly the digital input selected will have to be
programmed to execute this function (P618-P623, option 7; P420-P425 option 22). An enable left or enable
right signal is applied to define the direction in which the search for the reference point is to be performed. The
rate of reference point tracking depends on the current setpoint frequency. Reading in the point of reference is
ensured via one of the 6 digital inputs as well or else through one of the digital inputs of the customer
interface. For this to happen the digital input selected must have been assigned this function in the applicable
parameters (P618-P623, option 8; P420-P425 option 23).
The sequence of reference point tracking is as follows: When reference point tracking has been activated, the
the motor shaft will start turning in the direction the setpoint implies (enable right/left, setpoint sign +/-). The
direction of the motion is inverted by the signal received at the reference point input. When the axle has turned
so far as to trigger the reference point switch, the direction of motion is reversed. After the axle has moved out
of the operational range of the reference point switch again, reference point tracking continues until the
moment the next zero pulse is transmitted by the incremental encoder. If the position of the axle is such that
the switch responds the very moment reference point tracking is started, the axle will rotate in the inverse
sense right away. Likewise, after the axle has left the detection range of the switch, reference point tracking
will be completed when the next zero pulse of the incremental encoder is received.
This is when the current position is set to zero or to the value that has been set in the "offset" parameter. The
shaft remains in the new zero position until reference point tracking is terminated by the invalidation of the
"reference point tracking" signal. At the same time the setpoint position is set to zero in the "Position increment
array P610 = 1" position setpoint mode (relative positioning). When the frequency inverter has transferred the
reference point to memory, it will report that reference point tracking is complete through the action of multi-
function relay 3 or 4, provided that one of these has been associated with the "reference point" function in the
relevant parameter.
If following a "reference point found" signal the incremental shaft encoder fails to supply a zero pulse before
the shaft has completed 1.5 revolutions, the inverter will signal a "reference point" error (E14/E142).
If the incremental encoder is used without at the same time activating the "Save position" function (see
parameter 606)), the inverter will indicate "0" to be the actual position when it is switched on again, whereas if
the inverter was instructed to save the position, it will read out the value that was stored last.
When the display is reading out a "reference point" signal triggered by the relay, this means that a valid point
of reference has been identified. The relay drops out when the search for a reference point begins and picks
up again when this search has been completed.
If the "Save position" option (P606) has not been selected (which is usually the case), the relay drops out
when the inverter is switched on again.
If the "Save position" option has been selected though, the relay picks up immediately the moment the inverter
is switched on.
Reference point tracking can be interrupted by application of an "enable", a "quick stop", or a "cut off voltage"
command (without an error signal being activated).
3.2.1.2 Reset position function
As an alternative to reference point tracking as described above, one of the digital inputs provided on the
Posicon special extension can be assigned a position resetting function (P618 – P623, option 11). In that case
the input will be active all the time - whereas when executing the reference point tracking function it is
operative in an intermittent fashion - and will instantly set the actual position to zero or to the value selected in
the "offset" parameter as soon as the signal changes from 0 to 1, whether position control is ON or OFF. The
setpoint position in the digital mode 2 (position increments) is set to zero as well.
The repetition accuracy of referencing by means of the reset function falls short of the precision obtained by
reference point tracking as described above – it depends on the tolerance of the reference point switch and
the rate at which the operating threshold of the switch is approached. For many applications it is sufficient
though. Besides referencing can be carried out without interrupting position control.